Mark big and mach with ATTRIBUTE_UNUSED

Mark big and mach with ATTRIBUTE_UNUSED

Fix build on x86:

opcodes/disassemble.c: In function ‘disassembler’:
opcodes/disassemble.c:113:52: error: unused parameter ‘big’ [-Werror=unused-parameter]
 disassembler (enum bfd_architecture a, bfd_boolean big, unsigned long mach,
                                                    ^~~
opcodes/disassemble.c:113:71: error: unused parameter ‘mach’ [-Werror=unused-parameter]
 disassembler (enum bfd_architecture a, bfd_boolean big, unsigned long mach,
                                                                       ^~~~
cc1: all warnings being treated as errors

	* disassemble.c (disassembler): Mark big and mach with
	ATTRIBUTE_UNUSED.